1. Field of Invention
The present invention relates to a light guide plate and surface light source device provided with the light guide plate, and to an image display provided with the surface light source device. The present invention is applied to an image display provided with a member-to-be-illuminated (object-to-be-illuminated) such as image display panel backlighted by a surface light source device, and to a surface light source device and light guide plate used therein. A light guide plate, surface light source device and image display in accordance with the present invention may be used for image displaying in portable phones, portable terminals, electric dictionaries, various electronic devices or personal computers.
2. Related Art
It is known well to use liquid crystal displays for image displaying in portable phones, personal computers or other devices. A liquid crystal display is provided with a liquid crystal display panel as a member-to-be-illuminated. In many cases, backlighting is applied to the liquid crystal display panel and a so-called side light type surface light source device is utilized for backlighting.
FIG. 17 is a cross section view of a well-known fundamental structure of a side light type surface light source device, wherein a light path, which is depicted while primary light (i.e. light from a primary light source) enters into a light guide plate and then is emitted, is shown additionally.
Referring to FIG. 17, surface light source device 90 is provided with light source (primary light source) 91 and light guide plate 92. Light from light source 91 enters into light guide plate 9 92 sideways, propagating within light guide plate 9 92. On the way of such inner-propagation, inner-incidence to emission face 93 of light guide plate 92 occurs repeatedly. If inner-incidence angle is not greater than critical angle, some of inner-incident light outgoes from emission face 93. It has been proposed recently to aim to obtain uniformalized and highly bright emission from an emission face of a light guide plate by forming prismatic projections having emission promotion function on the emission face.
FIG. 18 shows an example thereof (first prior art), which is disclosed in Document 1 noted below. Referring to FIG. 18, surface light source device 112 is provided with light guide plate 102 and light source (primary light source) 110. Light guide plate 102 has an emission face 104 on which a great number of prismatic projection rows 113 running in a direction along incidence face 103 of light guide plate 102. These projection rows 113 promote outgoing of inner-propagation light in light guide plate 102.
Each projection row 113 is a prismatic projection having a triangle-like cross section, being provided with first slope 117 and second slope 118. First slope 117 is gently inclined so that light guide plate 102 decreases in thickness with an increasing distance from incidence face 103. On the other hand, second slope 118 is sharply inclined so that light guide plate 102 increases in thickness with an increasing distance from incidence face 103. On the other hand, Area occupied by first slope 117 is greater than area occupied by second slope 118. In addition, inclination angle θa of first slope 117 gets smaller gradually from incidence face 103 to distal end face 120. This and inclination of angle (wedge angle) α made by back face 107 of light guide plate 102 with respect to emission face 104 results in a cooperative effect such that emission from emission face 104 has a high and uniform brightness.
Next, FIG. 19 shows another prior art (second prior art), which is disclosed in Document 2 noted below. The second prior art aims to uniformalize and heighten emission brightness in generally the same manner as the first prior art (FIG. 18).
Referring to FIG. 19, surface light source device 130 employs light guide plate 131. Light guide plate 131 has back face 133 on which a great number of prismatic projection rows 132 are formed.
All projection rows 132 of light guide plate 131 shown in FIG. 19 gives the same inclination angle θa of first slopes 134 while intervals between two projection rows 132, 132 adjacent to each other varies in a suitable manner. This causes area ratio relation between first slope 134 and second slope 135 and height h of projection row 113 h to vary.
Document 1; International Laid-open pamphlet 2004/079258.
Document 2; Tokkai 2005-259361 (JP)
However, the above-mentioned first prior art fails to achieve sufficiently high and uniform emission brightness, for example, in cases as follows.
(1) Cases where inclination angle α of back face 107 of light guide plate 102 can not designed flexibly due to limitation, such as thickness, size or others, which light guide plate 102 is subject to.
(2) Cases where emission face 104 has short of emission ability depending on position thereon under condition such that inclination angle α is constant.
Although known are arts according to which emission face 104 or back face 107 is roughened in order to compensate short of emission ability, such arts causes, in general, emission face 104 to have a reduced emission directivity and, with the result that high brightness is hardly obtained.
According to the above-mentioned second prior art, intervals between two prismatic projection rows 132 adjacent to each other vary appropriately in order to uniformalize emission brightness, failing to enable individual projection rows to have an adjusted emission promotion ability due to absence of configuration variation thereof. Therefore, it is difficult to avoid emission face 104 from having control short or excess of emission brightness which can occur depending on position thereon,